1. Field of the Invention
This invention relates to a method of producing a synthesis gas comprising H.sub.2 and CO via the partial oxidation gasification of a sulfur- and metal-containing carbonaceous charge fuel. More particularly, this invention relates to an integrated method of (i) pretreating a sulfur- and metal-containing carbonaceous charge fuel to be gasified in a partial oxidation process, and (ii) removing sulfur-containing compounds from the tail gas stream exiting a Claus process sulfur removal unit employed to treat a portion of the gas stream produced by the partial oxidation process.
2. Information Disclosure Statement
It is well known by those skilled in the art that synthesis gas comprising primarily H.sub.2 and CO, together with various amounts of other gases, may be produced via the partial oxidation gasification of an ash-containing carbonaceous charge fuel at elevated temperatures. References disclosing such a process include coassigned U.S. Pat. Nos. 2,818,326 (Eastman et al.), 2,896,927 (Nagle et al.), 3,998,609 (Crouch et al.), and 4,218,423 (Robin et al.), all incorporated herein by reference. Such ash-containing carbonaceous charge fuels include crude residue from petroleum distillation and cracking processes, petroleum distillates, reduced crudes, whole crudes, asphalts, coals, coal tars, coal derived oils, petroleum cokes, shale oils, tar sand oils, and mixtures thereof. The production of synthesis gas from such charge fuels is described by, for example, coassigned U.S. Pat. Nos. 3,544,291 (Schlinger et al.), 3,976,442 (Paull et al.) and 3,996,026 (Cole), all incorporated herein by reference.
The abovedescribed ash-containing carbonaceous charge fuels ordinarily contain varying amounts of sulfur and sulfur compounds as well as metals and metal compounds, the metals including vanadium, nickel, and iron. The partial oxidation gasification of the charge fuel produces both raw synthesis gas comprising sulfur and sulfur compounds, particularly H.sub.2 S, as well as a molten slag comprising various other inorganic materials, including sulfur, sulfur compounds, metals, and metal compounds, the slag being derived from the ash portion of the charge fuel.
A common problem associated with the partial oxidation gasification process is the incomplete melting of the ash portion of the charge fuel. This causes the slag viscosity to increase, resulting in the accumulation of slag, together with its metal constituents, on the walls and refractory of the partial oxixdation gasiciation reactor. This accumulation of slag often leads to reactor operability problems as well as potential damage to the reactor refractory. It has therefore been proposed that various fluxing agents be employed as additives to the charge fuel to decrease the viscosity of the slag formed, thereby facilitating discharge of the slag from the reactor. For example:
Co-assigned U.S. patent application Ser. No. 947,122 filed Dec. 23, 1986 discloses the use of an iron-containing additive introduced into a petroleum coke or liquid hydrocarbon fuel partial oxidation feedstock, thereby producing a liquid phase washing agent that collects and transports vanadium and nickel impurities from the reaction zone;
Co-assigned U.S. Pat. No. 4,692,172 (Stellacio et al.) discloses a partial oxidation process which employs an iron sulfide containing material in admixture with a solid carbonaceous fuel, thereby lowering the melting point of the fuel ash to allow the process to be operated in the molten slagging mode at a lower temperature;
Coassigned U.S. Pat. No. 4,671,804 (Najjar) discloses an iron-containing additive, preferably iron oxide, for use in a partial oxidation gasification process. The additive reduces the viscosity of the molten slag, prevents the formation of toxic nickel subsulfide (Ni.sub.3 S.sub.2) in the slag, and additionally reduces the concentration of sulfur and sulfur compounds in the raw synthesis gas;
Co-assigned U.S. Pat. Nos. 4,668,428 (Najjar) and 4,668,429 (Najjar) disclose the use, respectively, of iron-containing and iron- and calcium-containing additives in the feedstock of a partial oxidation process to produce a liquid phase washing agent that transports at least a portion of vanadium-oxide compounds out of the reaction zone; and
Co-assigned U.S. Pat. No. 4,657,698 (Najjar et al.) discloses the use of an iron- and silicon-containing additive in the feedstock of a partial oxidation process to reduce the ash fusion temperature of the molten ash produced by the process.
Another problem inherent to the partial oxidation gasification process is that it is generally necessary to remove sulfur and sulfur compounds from the raw synthesis gas prior to use of the gas in chemicals production, power generation, and the like. Conventional processes employed in the removal of sulfur and sulfur compounds from synthesis gas (often called acid gas removal processes) are described, for example, in R. F. Probstein and R. E. Hicks, Synthetic Fuels 210-21 (1982), and include both liquid absorption and solid absorption techniques. Many processes are known which employ iron or iron compounds to remove sulfur or sulfur compounds from gas streams. For example:
"Status of Japanese R & D for IGCC Power Generation" (Terenuma et al.), presented at AIChE Summer Meeting, Boston, Mass. (August 1986) discloses at pp. 6-8 the use of a circulating fluidized bed of iron oxide particles to desulfurize raw synthesis gas generated from a coal gasification process;
U.S. Pat. No. 4,572,085 (Hepworth) describes a process for combusting sulfur-containing coal in the presence of a finely divided iron containing material to produce an essentially sulfur-free flue gas and a liquid iron oxysulfide slag;
API Petroleum Processes Abstract No. 86-1102a (June 2, 1986) describes E. German Pat. DD 229906, which discloses a process for desulfurizing a hot synthesis gas by contacting the gas with an iron-containing particulate bed, and thereafter passing the sulfur-containing waste gas produced therein to a lime reactor for contacting with lime and limestone, thereby producing an almost sulfur-free gas stream;
API Petroleum Processes Abstract No. 85-10667 (Apr. 22, 1985) describes Japanese Pat. No. 59230618, which discloses the use of double (metallic) oxide compounds where the metal may be Zn, Fe, Mn or Cu to remove H.sub.2 S from high temperature coal gas;
API Conservation, Transportation, Engineering and Storage Abstract No. 86-40675 (May 12, 1986) describes E. German Pat. No. 229039, which discloses the use of combustion plant ash fractions having high metal oxide content (e.g. Ca, Ms, Fe and Al oxides) to remove toxic components (e.g. SO.sub.2 and SO.sub.3) from flue gases; and
API Conservation, Transportation, Engineering and Storage Abstract No. 85-40537 (Apr. 8, 1985) describes USSR Pat. No. 1101286, which discloses a process in which a water slurry of ore enrichment waste material, e.g. pyrrhotile concentrate, tailings from Cu-Ni ore enrichment, or untreated magnetic concentrate, is contacted with SO.sub.2 -containing gas to remove the SO.sub.2 from the gas stream.
When a conventional acid gas removal process is applied to a sulfur-containing synthesis gas, an off-gas stream exiting the acid gas removal process remains containing substantial amounts of sulfur and sulfur compounds. This off-gas stream must therefore be further treated to remove sulfur and sulfur compounds. A Claus process unit is often employed to reduce the sulfur-containing compounds of the off-gas leaving the acid gas removal process to elemental sulfur. In this case, a tail gas exits the Claus unit. However, this tail gas generally still has too high a sulfur content to be released to the atmosphere; therefore a method for additional tail gas desulfurization must often be employed. Claus process units and tail gas clean-up methods are described, for example, in 22 Kirk-Othmer Encyclopedia of Chemical Technology 276-82 (3d ed. 1983). Such processes are often difficult and expensive to operate and maintain.
Given the abovedescribed problems, it would clearly be advantageous to employ a partial oxidation process whereby: (1) the accumulation of slag in the partial oxidation gasification reactor is minimized or eliminated; and (2) the need for Claus unit tail gas sulfur removal is eliminated.
It is the object of this invention to provide an integrated method of producing a synthesis gas via the partial oxidation gasification of a sulfur- and metal-containing carbonaceous charge fuel such that: (i) a fluid, molten slag comprising various metals and metal compounds is produced which flows easily from the discharge outlet of the partial oxidation gasifier reactor, thereby improving performance of the gasifier and preventing unwanted accumulation of slag and metals within the gasifier reactor; and (ii) the overall removal of sulfur-containing compounds from the synthesis gas generated by the partial oxidation process is made more simple and efficient, and less costly via an improved method of Claus process unit tail gas sulfur removal.
It is a feature of this invention that presulfided iron-containing compounds are employed as slag fluxing agents, thereby lowering the viscosity of the molten slag and more immediately encapsulating vanadium compounds and reacting with nickel compounds contained in the slag. It is another feature of this invention that Claus unit tail gas is employed to presulfide iron compound-containing particles which are thereafter used as a slag fluxing agent in a partial oxidation gasification reactor, thereby removing residual sulfur compounds from the tail gas.
It is an advantage of this invention that the presulfided iron-containing slag fluxing agent lowers the viscosity of the molten slag, thereby facilitating the free flow of slag from the partial oxidation reactor and avoiding slag accumulation within the reactor. It is another advantage of this invention that conventional Claus unit tail gas desulfurization processes, which are often difficult and expensive to operate and maintain, are replaced by the tail gas reactor of this invention, thereby combining tail gas desulfurization and iron compound fluxing agent presulfiding in a single operation.